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Understanding bonding can ensure the longevity of a hybrid treatment.
Of paramount importance in the design and fabrication of hybrid implant retained bars is service longevity. After nearly 20 years of requests to service these types of dental prosthetics, it is now obvious that after poor design, poor surface-to-surface bonding is a big reason for in-service failure. An understanding of products that promote bonding and recommended procedures can assure the promised life expectancy of hybrid treatment (Figures 1, 1a, 1b).
Bonding primers specifically formulated to bond acrylic or composite to alloy, ceramic, or hybrid resin surfaces tend to be marginalized, but they should be a standard protocol in hybrid fabrication. Their advanced chemistry provides a sound anchor when applying layered top coats and prolongs or potentially eliminates the possibility of subsequent delamination due to the micro flexures that occur during prosthetic function. Completely eliminating eventual fatigue to failure is not possible because of the individual flexural properties of the combination of material used in the overall fabrication. The aim, then, is to extend the functional life of the prosthetic beyond, if possible, the patient’s life expectancy (Figure 2).
In a typical titanium hybrid bar, combinations of overlays to consider are the following:
Products that this author uses are manufactured by GC Corporation. Other companies manufacture materials, but this article will explain the application techniques of GC products based on my success of use and familiarity (Figure 4).
For years, prior to hybrid fabrication, Metalprimer II has been the mainstay in my lab in the application of enhancing the bond of acrylic to chrome cobalt partial denture frames. I also now use an upgraded version, Metal Primer Z, in present metal bonding applications.
Figure 1. No primers were used in bonding. No remnant material is sticking to frame.
Figure 1a. Surface preparation of bar is missing. Design flaws are also evident.
Figure 1b. Surface reveals a brown coating on alloy. Indication of microleakage.
Figure 2. Elective stripping of bar when primers are used reveals a tenacious bond between alloy, opaque, and heat-cured polymethyl methacrylate. Removal entails cutting down into alloy. No flaking or microleakage is observed.
Figure 3. Every layer must have a complementary bonding primer.
Figure 4. Specialized to complement the bonding of various materials
Figure 5. Air abrasion and Metal Primer Z are prepared to accept opaque.
Figure 6. Opaqued, cured, coated with GC Acrylic primer and cured. Bar can now be replaced onto the processing model to proceed with acrylic adaption. If composite is to be applied to bar, substitute GC Composite primer or GC Ceramic II instead.
Figure 7. Gradia Plus, GC Lustre Paste, and Optiglaze
Figure 8. Visual step by step
In the past, I have used several types of “air-dry” opaque products. They work well to mask, but even when dry, they dissolve when they come in contact with monomer in uncured acrylic. This reveals itself many times after curing a case as a pink swirling effect in the acrylic. This is why I switched to the light cure form of opaque. It’s much more stable when contacting poured, packed, or pressed acrylic, and I feel it has a more sincere bond to the primer.
Prior to the manufacture of GC Acrylic primer, the protocol for bonding composite to a completed PMMA denture was to use GC Composite primer. Although composite primer is adequate for this application, this author recommends substituting the acrylic primer for the above-mentioned layering.
Although initially adding fabrication time and costs to the hybrid treatment, these bonding primer applications, coupled with sound design, are necessary additions to the hybrid bar fabrication workflow (Figure 8).